The Fundamentals Of Quantitative Analysis Of Electric Logs

All electric logs consist or two basic measurements. The first of these is a measurement of the electrical current variations that circulate in the borehole between the permeable beds and shales. The exploring electrode measures the electrical potential variations in the bore hole which are associated with these currents. Because these currents are generated by natural conditions and are in no way induced by the measuring equipment, the variations in electrical potentials are called "Spontaneous Potential," or "Self Potential." These are abbreviated to SP and are recorded on the electric log millivolts. The second basic measurement shown on an electric log is that of resistivity. Resistivity is the resistance of a unit volume of material to the flow or electrical current. The unit volume used in electric logging is that of a cube or one meter dimensions. Therefore, the unit of resistivity measurement is ohms per square meter per meter which is shortened to ohm-meters. The conduction or current within all earth materials, with the exception or certain metallic ores, is always associated with water. The current is carried by ions or salts in solution. (If this appears to contradict some papers on"conductive solids" it is only a contradiction of terminology. All shales, clays, and other porous matrices are excellent insulators when dry.) It would then logically follow that if more ions or salt per unit are present in the solution the resistivity will be smaller. However, these ions are able to move faster when the temperature or the solution is higher, primarily because the viscosity of the water is lower. As a result, in order to determine the resistivity of a solution, the concentration of salts dissolved in the water and the temperature of the solution must be known. Most of the waters encountered in well logging contain sodium chloride in solution. Fig. 1 shows the relation between the concentration or NaCl, the temperature, and the resistivity in ohm-meters.